Color image sensor cells are designed to be integrated into CCD (Charge-Coupled Device) or CMOS type digital cameras, for example cameras installed on portable telephones. The color image sensor cells are designed for coloring images acquired pixel by pixel through colored filter elements. Color image sensor cells comprise a metal level used particularly to form the fixed contacts necessary for the electrical connection of the component with external circuits or to an output from the housing in which it is mounted. The fixed contacts are actually conducting areas, usually square or rectangular that comprise a hole, in which a connection wire can be soldered. Conventionally, pure or doped aluminum is used to make the metal layer in which the fixed contacts are formed for soldering for a wire assembly. The color image sensor cell also comprises colored filter elements formed on a dielectric layer covering the metal level on which the fixed contacts are formed.
FIGS. 1 and 2 illustrate sequences in steps according to prior art used during the formation of a fixed contact and colored filter elements of a color image sensor cell. FIG. 1 thus shows a fixed contact 3 embedded in a passivation layer 1 formed from insulating oxide. An anti-reflecting layer 5, preferably made of titanium nitride TiN is advantageously provided to cover the area of the fixed contact 3. This titanium nitride layer originates from the use of the metal level exposure step particularly for the definition of fixed contacts. At this stage, titanium nitride is used for its optical properties to avoid having parasite reflections during photo exposure of the metal level on which the fixed contacts are defined.
The fixed contact 3 is etched in three steps; one etching step for the passivation layer 1, one etching step for the anti-reflecting layer 5, and one etching step for aluminum 3, called the over etch step, consisting of etching any residues remaining on the aluminum surface after the previous two steps. This sequence of steps results in an opening or hole 4 vertically in line with the fixed contact 3.
FIG. 2 shows the formation of colored filter elements on the passivation layer 1 of the cell. A first colored element 6 is formed by application of a colored resin layer, typically green, and by the formation of a pattern in this colored resin layer using conventional photolithographic exposure and development procedures. Similarly, the second and third colored elements 7 and 8 are defined starting from the deposition of a second layer of colored resin, typically blue, and a third layer of colored resin, typically red.
Conventional steps for the deposition, photolithographic exposure and development of a planarizing resin layer 9 are then applied so as to cover the three colored elements 6, 7 and 8. The three colored resins 6, 7 and 8 each have a different thickness, so that the deposit of the planarizing resin layer 9 covering the colored resins is necessary at this stage to enable the formation of micro-lenses in order to improve the sensitivity of the color image cell.
A step for formation of micro-lenses 10 on the planarizing resin layer 9 above the colored elements 6, 7 and 8 is then applied. This is done by depositing another resin layer called the micro-lens resin, for the formation of micro-lenses 10. The parts of the micro-lens resin layer that cover the regions not covered by colored elements 6, 7 and 8, and particularly the fixed contact, are then insolated and then removed by chemical development. The optical properties of the aluminum in the fixed contact have an incidence on the reflection of light during the step in which the micro-lens resin is insolated, therefore a strong exposure dose is necessary so as to completely develop the part of the micro-lens resin located above the fixed contact. The layer of micro-lens resin to be removed is exposed twice in order to avoid degrading the final dimensions of the micro-lenses 10 by an excessively long photo.
Thus, in a first exposure step of the micro-lens resin, a first mask is used so as to only insolate (i.e. expose) the part of the micro-lens resin located above the fixed contact, and a second mask is used in a second exposure step so as to insolate all parts of the micro-lens resin layer that will be removed for formation of the micro-lenses themselves. All that remains after a step in which the insolated resin parts are developed, are the resin contacts located above each of the colored elements 6, 7 and 8. Finally, a baking step is applied to make the resin contacts convex to form the micro-lenses 10.
The two-step exposure that overexposes only the micro-lens resin area located above the fixed contact provides a means of being sure that the area is sufficiently insolated so that it can be removed in its entirety. However, this is not advantageous in terms of manufacturing cost.
A final step called the “descum” step is then necessary, that consists of cleaning the surface of the fixed contact 3 with an oxygen plasma, so as to strip the surface of the fixed contact to remove any colored resin residues originating from the previous coloring steps.
However, there are several disadvantages in chaining steps according to prior art as has just been described with reference to FIGS. 1 and 2. In particular, the surface of the fixed contact 3 is degraded. The aluminum surface of the fixed contact 3 is actually degraded by the action of aggressive solvents used during photolithographic development steps designed to remove parts of previously insolated colored resins for definition of the patterns of the colored filter elements 6, 7 and 8. This degradation of the aluminum surface is a very serious problem when making electrical connections at the fixed contact. Furthermore, the last stripping step consisting of cleaning the surface of the fixed contact with oxygen plasma tends to degrade the physical dimensions of micro-lenses.
Thus, processes have been developed to improve protection for the integrity of the aluminum surface of the fixed contact in the context described above, in which coloring steps are used for the formation of colored filter elements. In particular, one existing method of improvement consists of depositing a layer for protection of the surface of the fixed contact once the etching steps have been completed, to open the fixed contact, as described with reference to FIG. 1.
Thus, as shown in FIG. 3, after carrying out the etching steps of the fixed contact 3, an intermediate step consists of depositing a protection layer 11, for example made of nitride, oxide or oxinitride. The purpose of the protection layer 11 is to protect the surface of the fixed contact 3 while the process to make the colored filter elements is being carried out. Thus, once the protection layer 11 has been deposited, the colored filter elements 6, 7 and 8 are formed on the protection layer 11 as already described above with reference to FIG. 2, and the layer of planarizing resin 9 is deposited so as to cover the colored filter elements. With reference to FIG. 4, the part of the protection layer 11 covering the fixed contact 3 is then etched so as to open the hole 4 vertically in line with the fixed contact 3. Advantageously, the planarizing resin layer 9 in this case acts as an etching mask. Once the protection layer 11 has been opened above the fixed contact 3, the micro-lens resin layer is deposited and the micro-lenses 10 are formed on the planarizing resin 9 above the corresponding colored filter elements 6, 7 and 8.
However, although this process described with reference to FIGS. 3 and 4 is capable of protecting the integrity of the aluminum surface of the fixed contact 3, it is not fully satisfactory. It requires the use of additional steps in the manufacturing process for deposition of the nitride protection layer and for opening it, which has the disadvantage that it increases manufacturing cycle times and therefore the cost of the final component.
According what is needed is a method and system to over come the problems and disadvantages of the prior art related to degradation of the aluminum surface of fixed contacts for a semiconductor color image sensor cell during the steps involved in making the colored filter elements, by making an improvement to protect the integrity of the aluminum surface of fixed contacts that is not excessively penalizing, particularly in terms of manufacturing costs.